Pulse width modulation (PWM) is a widely used technique in applications of power electronics where energy efficiency is important, such as DC power supplies and AC motor control. PWM circuits use power semiconductors as switches that are either on or off. When on, there is only a small voltage across the switch and therefore little power loss in it. When off, there is practically no current through the switch, hence almost no power loss in it. Thus, the switching process wastes little power.
A switching DC power supply is perhaps the simplest illustration of PWM. In such a supply, a load may be alternately switched between a source of unregulated DC voltage and ground. If the time during which the load is connected to the unregulated supply is equal to the time during which it is connected to ground, the average load voltage is equal to one-half of the unregulated voltage. If the time of connection of load to an unregulated supply exceeds the time of connection of the load to ground, average load voltage will exceed one-half the unregulated supply voltage, and conversely. If switching is done at a high frequency, the average voltage can be made to appear across the load by use of compact and simple filters which remove components of the load voltage at the switching frequency and its harmonics. A typical switching frequency is in the range 10-100 khz. From the foregoing, it may be seen that controlling the relative durations of switching to the unregulated supply and switching to ground is a means of controlling the average (or DC) load voltage.
In switching supplies, DC load voltage is usually sensed and compared with a required voltage. If there is any difference, the circuit acts to adjust the switching times in whatever direction will reduce that difference. Usually, in switching supplies and other applications of PWM, the total duration of the switching cycle (in the DC switching supply example, the sum of the time when the load is connected to the unregulated supply and the time when the load is connected to ground) is constant and the duty cycle, that is, the fraction or percentage of the total switching cycle time when a particular switch is closed, is varied.
A DC switching supply is relatively simple in that it produces a constant output voltage. It is, however, possible to use PWM to generate time varying output voltage merely by causing the switching duty cycle to vary with time, provided that the highest frequency at which the duty cycle is varied is low compared to the switching frequency so that output filtering can delete voltages at the switching frequency at its harmonics, leaving a time varying output voltage with the same waveform as that of the duty cycle variation. The identity of the duty cycle variation waveform with the output waveform is an important practical property of PWM, and follows from the linear relationship between duty cycle and average load voltage.
A DC switching supply is relatively simple in another way because its output is unipolar, i.e. either always positive or always negative. For other PWM applications such as AC motor control, a bipolar output voltage having no DC component is needed, and is usually obtained using a bridge circuit that alternately reverses the connection of the load to a DC supply, that is, during part of the switching cycle, one of the two load terminals is connected to the positive end of the DC supply and the other terminal is connected to the negative end, and during the rest of the switching cycle the connections are reversed. With such an arrangement, output voltage is zero if the duty cycle is 50%, and output voltage changes in polarity as the duty cycle varies from below 50% to above 50%.
For AC motor control, a sinusoidal output from a PWM controller is advantageous compared to other waveforms such as a square wave (see, for example, U.S. Pat. No. 5,165,005) because harmonics in the output waveform waste power in the motor and cause noisy operation as well as possible vibration problems. However, sine wave PWM is more complicated than square wave PWM, because it requires continuous variation of duty cycle above and below a mean value of 50%, while square wave PWM requires only two values of duty cycle, one less than 50% and the other greater. Furthermore, if the sinusoidal output is to have low harmonic content, the waveform of duty cycle variation must be accurately sinusoidal.
The main purpose of the present invention is to address a need for a simple, inexpensive circuit that will generate the switching waveforms required for precise sinusoidal PWM. Such a circuit has many potential applications, such as variable speed induction motor drive and variable amplitude linear motor drive, which applications at present may use square waveforms or relatively complex digital methods for producing sinusoidal waveforms. In some infrequent PWM applications, such as vibration control, duty cycle must vary with time non-sinusoidally. The methods and circuits of the present invention are usable in such application as well as for the special but much more common case of sinusoidal modulation.